Improve user ergonomics of brew link --overwrite help

[jasonkarns]

This PR makes the two-step: link --overwrite --dry-run followed by link --overwrite easier/faster for users. (Easy to leverage shell history to re-run the command without the --dry-run flag.)

Background

When a Keg is unlinked, brew-link gives a very helpful message for how to proceed: add the --overwrite flag.

For safety, it also recommends running in --dry-run mode first to see what would be deleted.

So a user's common flow would be:

1. run brew link foo
2. get error message with guidance
3. run brew link --overwrite --dry-run foo
4. inspect output
5. run brew link --overwrite foo

In this flow, steps 3-5 are likely very common to run back-to-back. Common enough that a user may use their shell history (up arrow) to re-populate their prompt with step 3's command, delete the --dry-run flag, and re-run.

The --dry-run flag needs to be removed from the command, of course. If it had been at the end of the command, it would make the subsequent modification easier.

Instead of "up arrow, left-arrow a bunch, then backspace over --dry-run, hopefully not backspacing over the formula name", it would be easier for the user if the dry-run flag were already at the end of the command. Then the user can "up arrow, backspace a few times and hit enter".

What's more, if the last arg were --dry-run, a more advanced bash user could even use !:- to re-run the link command with all-but-the-last-arg.

This change is admittedly quite trivial, and unlikely to heavily impact anyone's life. However, I find myself running these commands often enough in quick succession that it would be a nice improvement to make the "re-run for reals" step easier. (for users, like myself, that start by copy/pasting the command from brew's helpful message)

Additional

There is one existing reference to the string "link --overwrite --dry-run" in the "Common Issues docs.

If it's helpful to have that sample output updated to match, I'd be happy to do so.

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[MikeMcQuaid]

Seems fine, thanks @jasonkarns!